Wind turbines configured for use with their axis of rotation transverse to the wind direction are well known. In the case of wind turbines having their axis of rotation transverse to the flow of air, the axis of rotation is most conveniently (in terms of secure mounting and ability to accommodate different wind directions) oriented generally vertically, and such wind turbines are typically referred to as vertical axis wind turbines (or VAWTs) to differentiate them from horizontal axis wind turbines in which in use the axis of rotation is generally parallel to the wind direction. The rotor of a horizontal axis wind turbine must face either into or away from the direction of the wind and a yaw mechanism is required to rotate the rotor about the vertical axis of the tower to keep the rotor in proper alignment with the wind flow.
Vertical axis wind turbines (VAWTs) generally comprise a central shaft arranged vertically with respect to the ground and rotatably supporting a plurality of blades or vanes arrayed around the shaft and roughly perpendicular to the wind flow. Vertical axis turbines do not require a yaw mechanism to align the blades with the wind and the generator or other energy converter and related power transmission equipment may be mounted on the ground at the base of the turbine, potentially substantially reducing the complexity and cost of the installation.
The best-known types of VAWTs are the Savonius type (as shown in, U.S. Pat. No. 1,766,765, WIND ROTOR, issued 24 Jun. 1930 to Savonius, which discloses a Savonius-type turbine having a self regulating means) and the Darrieus type (as shown in U.S. Pat. No. 1,835,018, TURBINE HAVING ITS ROTATING SHAFT TRANSVERSE TO THE FLOW OF THE CURRENT, issued 8 Dec. 1931 to Darrieus). As indicated by the following exemplary patent documents, several different configurations of VAWTs have been developed.
U.S. Pat. No. 1,100,332, WINDMILL, issued 16 Jun. 1914 to Smith, discloses a two-level vertical-axis wind turbine, in which the top level has essentially Darrieus-type foils.
U.S. Pat. No. 3,941,504, WIND POWERED ROTATING DEVICE, issued 2 Mar. 1976 to Snarbach, discloses a vertical-axis wind turbine having three generally helical foils wherein the bottoms of the foils are spaced apart and the foils are twisted and configures such that the leading edges of the foils all meet at the tops of the foils.
U.S. Pat. No. 4,293,274, VERTICAL AXIS WIND TURBINE FOR GENERATING USABLE ENERGY, issued 6 Oct. 1981 to Gilman, discloses a helical Savonius-type turbine having means for varying the vane surface available for wind contact.
U.S. Pat. No. 4,365,934, WIND MACHINE, issued 28 Dec. 1982 to Mason, discloses a vertical-axis wind turbine, having blades pivotally mounted to enable them to swing out to catch wind, and to swing in to feather to reduce drag, as required during each rotation of the turbine.
U.S. Pat. No. 5,405,246, VERTICAL-AXIS WIND TURBINE WITH A TWISTED BLADE CONFIGURATION, issued 11 Apr. 1995 to Goldberg, discloses a Darrieus-type turbine with helical foils.
U.S. Pat. No. 6,428,275, HELICAL WIND ROTOR AND METHOD FOR MANUFACTURING SAME, issued 6 Aug. 2002 to Jaakola, discloses a helical Savonius-type turbine with a foil design utilizing a planar material.
U.S. Pat. No. 7,040,859, WIND TURBINE, issued 9 May 2006 to Kane, discloses a vertical-axis wind turbine having multiple generally vertically extending blades arrayed about its periphery and having a rounded cap.
U.S. Pat. No. 7,132,760, WIND TURBINE DEVICE, issued 7 Nov. 2006 to Becker, discloses a hybrid vertical-axis wind turbine comprising a pair of inner helical non-overlapping blades and a pair of outer longitudinally extending airfoil-type blades.
Publication No. US 2007/0104582, published 10 May 2007 (now U.S. Pat. No. 7,393,177, issued 1 Jul. 2008 to Rahai et al.), discloses a foil profile for a Savonius-type turbine.
Although, general developments in wind turbine technology may have some application to water turbine technology, given the differences in fluid density and operational environments, there has not been significant overlap in recent wind and water turbine developments. Further, as what is generally understood to be the primary benefit of transverse-axis turbines, alignment indifference, is less significant in water turbines, for which the predictability of the flow direction (in the case of rivers) or directions (in the case of tidal streams) simplifies alignment issues, there has been relatively little development of transverse-axis water turbines as compared to VAWTs. Water turbines are typically analogous to horizontal axis wind turbines in that they are configured such that in use the axis of rotation is preferably aligned with the flow direction of the water.